1. Field of the Invention
The present invention relates to a receiving apparatus for use with a radio communication system for transmitting for example an orthogonal frequency division multiplexing signal (hereinafter referred to as OFDM signal) and a receiving method thereof.
2. Description of the Related Art
In a recent radio communication system, a performance for transmitting a large amount of information such as picture data, in particular, moving picture data, as well as simple text data is required. Thus, in such a radio communication system, a highly efficient modulation system such as 16-ary QAM system or 64-ary QAM system should be essentially used. The 16-ary QAM system and 64-ary QAM system are modulation systems that modulate information with an amplitude or a phase that varies.
Consequently, such a receiving apparatus (hereinafter referred to as receiver) for use with a conventional radio communication system comprises an RF receiving means for receiving a radio signal modulated corresponding to the 16-ary QAM system or 64-ary QAM system and a demodulating means for coherent detection of the received signal and demodulating the received signal to an original data sequence.
However, in such a radio communication system, due to a multi-path propagation environment, the amplitude and phase of a signal waveform largely distort. Thus, when the receiver receives a distorted radio signal and coherently detects the received signal, a received data sequence that is different from the original data sequence is obtained.
To solve this problem, a technology that cancels or alleviates the distortions of amplitude and phase should be employed in the receiver.
For example, to alleviate the distortion of a channel, a technology for transmitting a reference signal from the transmitter and obtaining the channel distortion with the waveform of a reference signal received by the receiver is known.
When the channel distortion is obtained, the original transmitted signal can be estimated with the received signal. Thus, the receiver performance can be improved.
For example, in the environment of which the above-described orthogonal frequency division multiplexing (OFDM) signal is transmitted, the channel response (in the frequency domain) of the channel can be calculated with each received reference signal and each transmitted reference signal generated by the receiver. However, the channel frequency response contains a thermal noise and so forth generated in the receiver. To suppress the thermal noise, the calculated channel frequency response should be filtered in the frequency domain by a particular filtering means.
When the channel frequency response is filtered in the frequency domain, the Gaussian noise component can be suppressed. Thus, the channel frequency response calculated by the receiver can be approached to the channel frequency response with no Gaussian noise.
In this case, a filtering means with a bandwidth corresponding to the channel frequency response should be used.
In other words, when the bandwidth of the filtering means is designated to a value larger than a proper value, the channel frequency response that has been filtered still contains a large noise component. In contrast, when the bandwidth of the filtering means is designated to a value smaller than the proper value, the channel frequency response gets distorted. Thus, in both the cases, good receiver performance cannot be obtained.
However, in the receiver (receiving apparatus) for use with the conventional radio communication system, the channel frequency response largely depends on the propagation environment of a radio wave. When the moving receiver receives a radio signal, since the propagation environment of the radio wave largely varies, it is almost difficult to use a filtering means with a bandwidth suitable for the propagation environment of the radio wave that varies in such a manner.